In known processes for isomerising aromatic compounds containing eight carbon atoms, a feed, which is generally depleted in para-xylene with respect to the thermodynamic equilibrium of the mixture (i.e., where the para-xylene content is substantially lower than that of a mixture at thermodynamic equilibrium at the temperature under consideration, that mixture being constituted by meta-xylene, ortho-xylene, para-xylene and ethylbenzene) and generally rich in ethylbenzene with respect to that same mixture at thermodynamic equilibrium, is introduced into a reactor containing at least one catalyst, under suitable temperature and pressure conditions to obtain at the outlet from that reactor a composition of aromatic compounds containing eight carbon atoms which is as close as possible to the composition of that mixture at thermodynamic equilibrium at the reactor temperature.
From that mixture, the para-xylene and possibly ortho-xylene are separated out since they are the isomers which are sought as they are of importance, in particular to the synthetic fibre industry. The meta-xylene and ethylbenzene can then be recycled to the isomerisation reactor inlet so as to increase the production of para-xylene and ortho-xylene. When ortho-xylene is not to be recovered, it is recycled with the meta-xylene and the ethylbenzene.
Reactions for isomerising aromatic compounds containing eight carbon atoms per molecule, however, encounter a number of problems caused by secondary reactions. Thus in addition to the principal isomerisation reaction, hydrogenation reactions are observed such as hydrogenation of aromatic compounds to naphthenes, also naphthene ring opening reactions which lead to the formation of paraffins containing at most the same number of carbon atoms per molecule as the naphthenes from which they originate. Cracking reactions are also observed, such as paraffin cracking which leads to the formation of light paraffins typically containing 3 to 5 carbon atoms per molecule, and dismutation and transalkylation reactions which lead to the production of benzene, toluene, aromatic compounds containing 9 carbon atoms per molecule (for example trimethylbenzenes) and heavier aromatic compounds.
The aggregate of such secondary reactions substantially affects the yields of desired products.
The quantity of secondary products formed (primarily naphthenes containing 8 carbon atoms, paraffins containing 8 carbon atoms, benzene, toluene, and aromatic compounds containing 9 or 10 carbon atoms per molecule) depends on the nature of the catalyst and the operating conditions of the isomerisation reactor (temperature, partial pressures of hydrogen and hydrocarbons, feed flow rate).
The skilled person is aware that secondary reactions increase when the para-xylene content in the reactor is close to the amount of para-xylene at thermodynamic equilibrium under the given temperature and pressure conditions.
Optimising the operating conditions and optimising the formulation of the isomerisation catalyst can increase the para-xylene yield, but cannot overcome the losses. Further, research to obtain new catalysts is a long and expensive business.